Setting well casings



April 1956 A. D. GARRISON 2,742,090

SETTING WELL CASINGS Filed July 25, 1952 INVENTOR. @LLEND. GAPp/so/v ATTORNEY SETTENG were CASINGS Allen D. Garrison, Eouston, Tex, assignor to Texaco Development Corporation, New York, N. ii, a cor-pd ration of Delaware Application July 25, 1952, Serial No. 300,866 18 Claims. (Cl. 16622) The present invention relates to setting a well casing within the bore of a subsurface well or bore hole, and, more particularly, to so-called temporary cementing of the casing string in the bore hole.

The present invention contemplates injecting into the space exteriorly of the casing string, namely, between its outer surface and the surface of the bore hole, a reversible gel-forming fluid of slow setting rate and high final gel strength, efiective to seal, protect and reinforce the walls of the bore hole, but capable of being readily degelled to its original fluid state to permit removal of the casing.

In accordance with the present invention, therefore, the casing is cemented in the bore hole by means of a colloidal thixotropic clay dispersion in an aqueous fluid capable of forming a reversible gel in the bore hole about the outer surface of the casing string after it has been pumped into place. The gel-forming dispersion possesses a sufiiciently high final gel strength and rigidity to provide an adequate supporting bridge between the casing and the walls of the bore hole which adequately seals, supports and reinforces the walls of the bore hole.

An essential property of the high strength gel is its capacity to degel to a fluid state by mechanical disruption, as for example, by forcing the casing into longitudinal or rotary motion. As a result, a temporarily set casing is readily recoverable when it has served its purpose.

Another important property of the gelling fluid contemplated in accordance with the present invention is its abnormally slow gelling rate or gel setting rate which is important from the standpoint of so delaying the effect of the abnormally high final gel strength or rigidity, as to permit injection of the gel-forming fluid into operative position about the casing and enable removal of the casing.

More specifically, the present invention contemplates cementing with a colloidal clay suspension as above, which, during injection or circulation into a place, inherently and automatically retains a flowing viscosity sufliciently low to be handled or circulated by the pumps and circulating means, and which, when left in a state of quiescence after injection, as above, gels to a 24-hour gel strength in excess of 100 pounds per 100 square feet, and preferably at least 200 pounds on the same basis. Actually, it is contemplated employing gels of as high as, for example, 300700 pounds gel strength and higher, all of which readily degel to form low viscosity fluids. Gels of such strength are adequate to support and seal formations normally encountered.

It is also contemplated employing high gel strength colloidal suspensions, as above, the gelling rate of which is abnormally low, namely, a rate such that at least two hours, and preferably three hours, are required to attain one-half the 24-hour gel strength.

Also, the colloidal suspension contemplated desirably possesses a flowing viscosity suiflciently low to be pumped or circulated by conventional equipment, preferably a viscosity not substantially in excess of 50 centipoises at 600 Z,742,fl0 Patented Apr. 17, 1956 R. P. M. in a Stormer viscosimeter; that is, up to about 7080 centipoises on the same basis.

The present invention is of particular advantage, as previously indicated, from the standpoint of removal and recovery of extensive lengths of valuable casing which are frequently lost when cemented in place by an irreversibly setting, hydraulic cement for the purpose of testing producing strata and the like. Even when the hydraulic cementing is restricted only to its lower portion, the casing, when cut and freed from the cemented section, is frequently frozen in place by collapse of the bore hole walls against the casing so that recovery is diflicult or impossible.

In accordance with the present invention, however, the abnormally high strength gel about the casing supports and reinforces the Walls of the bore hole, preventing the dislocation of rock fragments and supporting against gravitation or settling any dislodged formation fragments which otherwise tend to pack about the casing and bind it tightly against recovery.

Obviously, also, the high strength gel prevents migration of subsurface fluids along the exterior of the surface, and, upon application of rotational or pulling strains,

degelling promptly occurs, enabling withdrawal of the cas- Advantageously, a very short section at the lower end of the casing may be embedded in an irreversibly setting hydraulic cement to effect a permanent seal at this point, in which case the limited string of casing involved may be broken loose, or may be cut otf from the remainder of the casing thereabove to facilitate recovery.

Previously proposed, aqueous clay suspensions, as used for drilling fluids, and having suflicient fluidity to permit pumping and circulation even when compounded with the best colloidal clays available, will not gel to a gel strength of the order required of an effective cementing material in accordance with the present invention. Increasing the amount of highly colloidal clay in the aqueous suspension increases final gel strength, but the viscosity is also increased so that pumping and injection of the fluid becomes impractical, and likewise the gelling rate is objectionably increased.

The present invention contemplates provision of a thixotropic clay suspension having an abnormally slow and prolonged gelling process such that relatively large amounts of colloidal clay can be incorporated in the fluid while still retaining a low circulating viscosity, the high clay concentration resulting in a strong cementing gel after a suitable period of quiescence, which easily reverses to its original fluid state simply by overpowering its gel strength and forcing it back into a condition of fluid flow.

In its broadest aspect, the present invention contemplates the setting of well casings by means of a thixotropic fluid of the foregoing character irrespective of its method of production or compounding, based on the fact that the requisite physical properties may be attainable by a number of expedients.

It is preferred, however, to prepare such thixotropic cementing fluids by the use of a high yield colloidal clay in substantial concentration, adjustment of exchangeable ions in the clay to favor low gelling rate and low fluid viscosity, by incorporation of efiective amounts of a slowgelling agent and by maintenance of a pH in the range of about 9.S-11.0.

For example, it has been found that the gelling rate has a minimum value at a pH in the range of 9.5-1l.0. But even at pH values of 9.511.0, clay suspensions composed of the best natural colloidal clays do not yield simultaneously the requisite combination of properties as regards final gel strength and slow gelling rate meeting the requirements of a reversible cementing material in accordance with the present invention.

It is, therefore, contemplated employing suspensions of high quality, high yield clay, modified to exhibit the desired properties. For example, a material alteration in clay properties may be realized by conversion of natural high yield, colloidal clay mineral, such as bentonite, to a more completely sodium-saturated condition than is normally found in nature. This follows from the fact that the mineral, as is known, comprises anionic micelles together with adsorbed alkali and alkaline earth and hydrogen cations, usually, for example, calcium and magnesium cations. I have found that the adsorbed alkaline earth metal cations and hyddogen cation (e. g. Ca|+, Mg++, H+) contribute to relatively high gelling rates, and that substitution of these by alkali metal cations, such as the sodium, lithium or potassium ions, results in a material decrease in gelling rate without lowering the final gel strength.

The substitution of the adsorbed divalent clay cations and of the hydrogen cation by monovalent alkali metal ions, such as sodium, is herein referred to as conversion to the alkali metal form of salt or clay, clays thus converted predominantly or substantially completely to the alkali metal form favoring high gel strength when colloidally dispersed in large amounts, without contributing to high gelling rates.

In general, the H+ ions present in the clay are inherently eliminated at the relatively high ranges of pH aforementioned (9.5-1 1.0)

Conversion of the clay mineral as above may be effected by electrodialysis followed by readjustment of the pH to the alkaline range with pure sodium, lithium or potassium hydroxide. Alternatively, calcium and magnesium ions of the original clay may be removed by exchange, for example, with ion exchange compounds such as sodium metasilicate, sodium pyrophosphate, sodium metaphosphate, sodium triphosphate, and the like.

The gel rate is further reduced without impairing final gel strength by inclusion of small amounts of a gelslowing agent, such as sodium tannate, sodium gallate or alkali quebracho, and the like, which are usually polyhydroxy organic compounds of the plant tannin type.

'It is particularly contemplated adopting a combination of the above treatments with appropriate selection of the clay and clay particle concentration to enable realization of the required combination of properties.

It is to be understood, of course, that the contemplated reversible cementing fluid is a non-settling, colloidal gel, and to this end is composed of hydrated clay not less than 50% of whch is subdivided into particles as small as cm. in diameter.

Such a clay is usually defined in the drilling art as a high yield clay, and is herein defined as one which, when made up wtih water alone will produce 50 or more barrels of 30 centipoise (600 Stormer) fluid per ton of clay.

Modification of a high yield clay in accordance with the foregoing principles may be illustrated as follows:

A. A suspension composed of 5.5% of a sample of a high yield clay, California bentonite, in water possessed a final gel strength of about pounds per 100 square feet, at a pH of 9.2. It attained one-half this gel value in less than two minutes in quiescent state.

B. When, however, this bentonite was electrolized to apH of about 2.8, then raised to a pH of 9.9 with pure sodium hydroxide, the final gel strength was 21 pounds per 100 square feet and required 30 minutes to reach one-half this value.

C. Starting with 5.5% of California bentonite and adding .l% Na tannate, only, the respective properties are as follows: final gel strength 22 pounds per 100 square feet and 2 hours required to attain one-half final gel strength.

The above examples B and C show the effect on gel rate and final gel strength of using independently, first,

the principle of removing divalent and hydrogen ions and substituting sodium ions, and, second, the effect of introducing a gel-slowing agent.

Since, however, the final gel strength of these fluids does not approximate the requirements of the present invention, the following examples are presented to illus trate a fluid meeting these requirements:

Example 1 Example 2 Another fluid suspension was prepared containing 100 grams Wyoming bentonite, 2 grams gallic acid, 3.2 grams sodium meta silicate, water to make a liter, and enough potassium hydroxide to raise the pH value to 10.5. This fluid had approximately 68 centipoises viscosity at 600 R. P. M. (Stormer). On standing in the quiescent state, the fluid gelled slowly to approximately 620 pounds per 100 square feet gel strength, and attained one-half of this 24-hour value in about 3 hours and 20 minutes.

On the basis of either of the above examples, it is possible to realize a further increase in the final gel strength by further increasing the concentration of colloidal clay particles. Such treatment is normally accompanied by an increase in the viscosity and, therefore, is limited by the equipment available to circulate fluid into operative position about the casing.

However, since the previously mentioned gel-slowing agents have the property of substantially reducing gel rate without materially lessening the final gel strength, if their effective concentration is increased, the solid, colloidal particle concentration may be raised materially to realize even higher final gel strength values without causing the gelling rate to become too high.

One method of setting a casing in accordance with the present invention is illustrated in the present drawing wherein Figure l is a vertical sectional view taken through a well during the injection of the reversibly gelling material about a casing, and Figure 2 is a subsequent view after landing the casing in final position.

In such an operation, the casing 10 may be run into a well bore 12 extending into formation 16 to a position, as indicated, with the casing shoe 14 just above the bottom of the hole. For example, the casing may be suspended so thatthe shoe 14 is 2-60 feet above the lower extremity of the bore hole. A string of tubing 18 which carries the cementing material is run centrally through the casing as indicated, terminating close to the bottom of the casing 10 where it passes through a packer 20. The packer, as indicated, occupies the space between the exterior of the tubing and the interior of the casing, and accordingly seals the casing.

A reversible slow-gelling fluid of the characterhereinbefore described is introduced 'into the tubing 18 through well head 22 via pipe 24 controlled by valve 28. The injected material is identified by the reference numeral 26 and, as shown, passes around the lower extremity of the shoe 14, and up along the outside of the casing. After a sufficient amount of this material'has been injected under pressure to fill the annular space about the casing, a relatively small slug of hydraulic cement slurry may be injected through pipe 18 and placed about the shoe 14 and the lower end of casing 10. The valve 28 is; closed and the casing lowered to set or land the shoe 14 firmly upon the formation at the bottom of the hole. At this point, the injected gel-forming fluid 26 behind the casing is, of course, sealed, in situ, as indicated in Fig. 2, and can no longer flow out of the space between the casing and the walls of the bore hole. The reference numeral 30 in Fig. 2 identifies the slug of hydraulic cement slurry which may be driven by a following stream of drilling mud or any other suitable fluid 32. Preferably the well is closed in for a period of 24 hours to permit the gel to approach its final maximum gel strength and to set the hydraulic cement. Then tubing 18 and packer 20 are withdrawn and further drilling or testing proceeds in the usual manner.

Advantageously, a predetermined amount of the reversible gelling fluid is placed or driven into position behind the casing as above. The following or succeeding fluid may be separated from the cementing fluid by means of a conventional, movable plug or packer. Such a plug moves freely through the pipe to separate the respective fluids and to prevent intermixing. Also, where a slug of hydraulic cement is to be placed about the shoe, this may be separated from the reversible gel by means of movable plugs.

In such case, tube 18 may be omitted and the cementing fluid passed directly down the casing in a predetermined amount.

As is obvious from the foregoing, reduces the amount of hydraulic cement required, but a more important advantage results from the easy recovery of the gel-cemented portions of the casing. In other words, upon moderate agitation, the gel proceeds to reverse, forming a relatively low viscosity fluid which releases the casing and can be circulated out of the hole by Water or ordinary drilling mud.

Obviously, many modifications and variations of the invention as herein set forth may be made without departing from the original spirit and scope, and, therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. The method of setting a well casing within the bore hole of a subsurface well which comprises injecting behind said casing between the outer surface of said casing and the wall of said bore hole a thixotropic gelforming fluid having a 24-hour gel strength in excess of 100 pounds per 100 square feet, and a gel rate such that it requires at least two hours to attain one-half of the 24-hour gel strength, said fluid consisting essentially of a high yield, predominantly alkali-metal base clay suspended in water, said clay being present in said fluid in an amount sufficient to attain said 24-hour gel strength, and an amount of a polyhydroxy plant tannin gel-slowing agent suflicient to attain said gel rate, the pH of said fluid being adjusted in the range 9.5-11.0.

2. The method of setting a well casing within the bore hole of a subsurface well which comprises injecting behind said casing between the outer surface thereof and the wall of said bore hole a thixotropic gel-forming fluid having a flowing viscosity sufliciently low to be pumped into said position and having a 24-hour gel strength in excess of 100 pounds per 100 square feet and a gel rate such that at least two hours are required to obtain onehalf the 24-hour gel strength, said fluid consisting essentially of a high yield, colloidal, predominantly alkalimetal base bentonitic clay suspended in water, said clay being present in said fluid in an amount sufiicient to attain said 24-hour gel strength, an amount of a polyhydroxy plant tannin gel-slowing agent sufficient to attain said gel rate and an amount of an alkali-metal hydroxide suflicient to adjust the pH of said fluid in the range 9.5-11.0.

3. The method according to claim 2 wherein the 24- hour gel strength is at least 200 pounds per 100 square feet.

4. The method according to claim 2 wherein the 24- the present invention hour gel strength is in the range of about 300-700 pounds per square feet.

5. The method according to claim 2 wherein the gel rate of said fluid is such that at least about 3 hours are required to reach the 24-hour gel strength.

6. In the setting of a well casing within the bore hole of a subsurface well the step which comprises injecting behind said casing between the outer surface of said casing and the wall of the bore hole a thixotropic gelforming fluid having a 24-hour gel strength in substantial excess of 100 pounds per 100 square feet and suflicient to form an efiective supporting bridge between the easing and the wall of said bore hole and having a gel rate such that said fluid requires at least two hours to attain one-half of the 24-hour gel strength, said fluid having a pH in the range 9.511.0 and consisting essentially of a high yield, predominantly alkali-metal base colloidal hydrated clay having a particle size such that not less than 50% of the clay particles are as small as 10* cm. in diameter, said clay being present in said fluid in an amount sufficient to attain said 24-hour gel, and a poly hydroxy plant tannin gel-slowing agent in an amount sufiicient to attain said gel rate.

7. A thixotropic gel-forming fluid having a 24-hour gel strength in excess of 100 pounds per 100 square feet and a gel rate such that it requires at least two hours to attain one-half of the 24-hour gel strength, consisting essentially of a high yield, colloidal predominantly alkalimetal base clay suspended in water, said clay being present in said fluid in an amount sufiicient to attain said gel strength, and an amount of a polyhydroxy plant tannin gel-slowing agent sufficient to attain said gel rate, the pH of said fluid being adjusted in the range 9.5'11.0.

8. A composition in accordance with claim 7 wherein said clay is a high yield, bentonitic clay which has been subjected to ion exchange so that substantially all of its adsorbed alkaline earth divalent cations and hydrogen cations have been replaced by an alkali metal ion.

9. A composition in accordance with claim 7 wherein said clay is a bentonitic clay which has been subjected to ion exchange with an alkali metal ion exchange compound.

10. A composition in accordance with claim 7 wherein said clay is a bentonitic clay which has been subjected to ion exchange with a compound selected from the group consisting of sodium metasilicate, sodium pyrophosphate, sodium metaphosphate and sodium triphosphate.

11. A composition in accordance with claim 7 wherein said gel-slowing agent is a compound selected from the group consisting of sodium tannate, sodium gallate and alkali quebracho.

12. A thixotropic gel-forming fluid having a 24-hour gel-strength in excess of 100 pounds per 100 square feet and a gel rate such that it requires at least two hours to attain said 24-hour gel strength, consisting essentially of an aqueous suspension of a high yield predominantly alkali-metal base clay, in the presence of gallic acid, sodium metasilicate and an alkali metal hydroxide, said materials being present in the proportions 100:l2:3.2:1 parts by weight, respectively, the resulting fluid having a pH in the range 9.5-11.0, said clay being present in said fluid in an amount suflicient to attain said 24-hour gel strength.

13. A composition in accordance with claim 7 wherein said gel-slowing agent is sodium tannate.

14. A composition in accordance with claim 7 wherein said gel-slowing agent is sodium gallate.

15. A composition in accordance with claim 7 wherein said gel-slowing agent is alkali quebracho.

16. A method in accordance with claim 1 wherein said gel-slowing agent is sodium tannate.

17. A method in accordance with claim 1 wherein said gel-slowing agent is sodium gallate.

8 18. Amethod in accordance with claim 1 wherein said 2,213,038 David Aug. 27, 1940 gel-slowing agent is alkali quebracho. 2,213,039 David Aug. 27, ,1940 r 2,582,909 Laurence Jan. 15, 1952 References (fired in the file of this patent 7 OTHER REFERENCES 0 UNITED STATES PATENTS Rogers: Composition and Properties of Oil Well Drill- 1,563,520 Owen Q Dec. 1, 1925 ing Fluids, 1st ed., 1948, pages 448 and 449. 2,104,488 Kennedy et a1 Ian. 4, 1938 Rogers: Composition and Properties of Oil Well Drill- 2,167,455 Hirschman July 25, 1939 ing Fluids, 1st ed., 1948, pages 281, 299, 315, and 316. 2,193,144 Rymal Mar. 12, 1940 V 

